With the growth of the wireless communications industry, wireless communications protocols become more sophisticated and demanding in their requirements for complex modulation schemes and narrow channel bandwidths. The ultimate goal is to encode as much digital information as possible in a given channel. One such modulation scheme for encoding digital information is polar modulation. Polar modulated RF transmitters utilize both amplitude modulation (AM) and phase modulation (PM) to maximize the amount of information that can be encoded with minimum bandwidth. By using multiple combinations of phase and amplitude modulation, multiple digital bits of information can be represented. Large signal amplitude modulation allows several distinct levels of modulation with adequate noise margins for reliable encoding of digital data.
In a polar modulated system, AM can affect proper operation of an RF power amplifier in an RF communications terminal due to non-linearities of the RF power amplifier. As a result, the RF power amplifier may have amplitude based distortions. One type of amplitude based distortion is called AM to AM distortion, which may be present when the RF power amplifier has a non-linear gain with varying output power. Another type of amplitude based distortion is called AM to PM distortion, which may be present when an amplitude signal that controls the RF power amplifier causes unwanted phase components to be created due to the non-linearities of the RF power amplifier.
To compensate for AM to AM distortion, the amplitude signal that controls the RF power amplifier may be pre-distorted using AM to AM pre-distortion. However, to determine the appropriate AM to AM pre-distortion that will correctly compensate for the AM to AM distortion of the RF power amplifier may require a calibration process to adequate characterize the AM to AM distortion and then determine the appropriate AM to AM pre-distortion to compensate for the AM to AM distortion. Similarly, to compensate for AM to PM distortion, a phase modulated RF signal to the RF power amplifier may be pre-distorted using AM to PM pre-distortion. However, to determine the appropriate AM to PM pre-distortion that will correctly compensate for the AM to PM distortion of the RF power amplifier may require a calibration process to adequate characterize the AM to PM distortion and then determine the appropriate AM to PM pre-distortion to compensate for the AM to PM distortion. Costs of calibration processes in production environments tend to be directly related to calibration set-up and execution times. Thus, there is a need for amplitude based pre-distortion calibration of an RF communications terminal that minimizes calibration set-up and execution times.